The present invention generally relates to a thrust ball-bearing, and more particularly to a thrust ball-bearing in which a plurality of balls are held in predetermined positions by a retainer situated between two plates.
A description will be given of an example of a conventional thrust ball-bearing with reference to FIG. 1 and FIG. 2. FIG. 1 is an enlarged cross sectional view of a disk driving motor in a conventional FDD (Floppy Disk Drive) apparatus, and FIG. 2 is an exploded view of a bearing portion of the disk driving motor shown in FIG. 1.
A spindle shaft holder 6 is fixed on a chassis 1 and a plurality of coils 4 radially surround the shaft holder 6 on the chassis 1. A spindle shaft 2 is fixed, by press fitting, to a turn-table 3 that rotates with a disk loaded thereon. A magnet 5 is provided on an inner surface of the turn-table 3 in a position such that the magnet 5 faces the coils 4. Additionally, one end of the spindle shaft 2 is inserted into the spindle shaft holder 6 and thus the turn-table 3 is rotatably supported on the chassis 1.
A thrust force applied in an axial direction of the spindle shaft 2 of the turn-table 3 is carried by a ball bearing 11 which comprises plates 7 and 10, a retainer 8 and a plurality of balls 9. Each of the balls 9 are rotatably retained by being situated inside equally spaced holes formed on the annular retainer 8. The retainer 8 is sandwiched between the two annular plates 7 and 10 from the upper side and the bottom side. Portions of each ball 9 protrude from the upper surface and the bottom surface of the retainer 8 such that the plates 7 and 10 make contact with the balls 9; thus the plates 7 and 10 are rotatable relative to each other via the balls 9.
In such a ball bearing structure, it is usual to apply an amount of grease on a surface of the balls 9 at assembly time so that friction between the balls and plates is reduced and the bearing can maintain a smooth rotation.
However, since the grease applied to the ball rotates together with the ball, the grease is scooped by an edge of the hole that retains each ball and the grease is collected in portions between the retainer 8 and the plates 7 and 10. A centrifugal force is exerted on the grease, which force is generated by a rotation of the bearing. Since the grease gradually moves outwardly from a portion where the ball 9 makes contact with the plates 7 and 10, the lubrication of the ball is reduced.
Ball-bearings such as above mentioned are generally adopted for use in home appliances to which home appliances periodical application of grease is not provided as in the case of industrially used appliances. Therefore, when an amount of grease decreases, from the surface of the ball 9, after a considerable time of use, friction between the ball 9 and the plates 7 and 10 increases.
As mentioned above, when the lubrication of the ball is decreased, wear of the balls and plates is increased and thus there is a problem in that a life of the ball bearing becomes short.